The US senator from New Jersey also wants the Justice Department to send federal authorities to the protest site to ensure demonstrators can exercise their First Amendment rights.

US Senator Cory Booker on Friday called on the Department of Justice to investigate reports of what he said were inappropriate police tactics at the Dakota Access Pipeline near the Standing Rock reservation in North Dakota.

In addition to the use of police dogs, rubber bullets, and tear gas — which have been deployed against protesters for the past several months — law enforcement has recently begun to use water cannon on demonstrators who oppose the construction of the 1,172-mile long pipeline, an especially controversial measure given the colder temperatures in North Dakota.

Since August, thousands of protesters have arrived at the Standing Rock site to stand with Native Americans in opposing the pipeline, which is designed to carry 20 million gallons of oil across the Midwest every day. The Army Corps of Engineers on Saturday told protesters to leave the camp by Dec. 5.

In a letter to Attorney General Loretta Lynch and her deputy, Vanita Gupta, Booker called upon the DOJ to “promptly and thoroughly investigate all credible reports of inappropriate police tactics,” and to “send federal monitors to Standing Rock to ensure that protesters can peacefully assemble and exercise their First Amendment rights” if they had not already done so.

The US senator from New Jersey said he was “deeply troubled by this tense situation, and particularly by reports indicating that law enforcement may be responding to peaceful protestors near Standing Rock with overly aggressive tactics.”

Booker also expressed concern over reports of “unconstitutional conditions of confinement,” including holding demonstrators in overcrowded cages “akin to dog kennels, on bare concrete floors without access to medical care.”

He maintained that it is equally important to ensure the safety of the officers deployed to monitor the protests, stating that an “assault on a police officer is an assault on all of us and should not be tolerated.”

At least 33 people protesting the construction of the oil pipeline were arrested for “criminal trespass” at a shopping mall in Bismarck, North Dakota, on Black Friday.

Bismarck police said they responded to calls from Kirkwood Mall after about 100 protesters gathered there on one of the busiest shopping days of the year.